1. Field of the Invention
The present invention relates to a system and method for hydrophones for sensing acoustic pressure waves. In particular the invention relates to a technique for obtaining uniform buoyancy and increased sensitivity utilizing a hollow microsphere loaded reaction injection molding polyurethane material.
2. Description of the Related Art
Current methods of constructing non-oil-filled or solid towed arrays have proven to be prohibitively expensive and complex due to their method of construction, which adversely affects reliability. Such techniques are described in U.S. Pat. Nos. 5,774,423; 5,361,240; 5,883,857; and 4,733,378. As a result, solid towed arrays, while demonstrating substantial advantages over oil filled arrays, have met with limited acceptance within the seismic and surveillance communities due to their high price and unreliability. Thus there is a need for a technology which addresses the primary causes of high cost and low reliability with a unique method of construction that eliminates the primary cause of failure and reduces the labor required to construct the array.
The primary cause of failure of solid towed arrays results from the requirement to make electrical connections for each of the numerous hydrophones that make up a single channel in an array. In the case of the cable described in U.S. Pat. No. 5,883,857, the method of construction calls for a discrete entry into the primary cable in order to connect each hydrophone. Prior methods of construction have utilized a xe2x80x9cfloatationxe2x80x9d cable design that extrudes a buoyant material, such as foamed polyethylene, over an inner jacketed cable, which is then covered with an outer extrusion of polyurethane. The primary problem with this design is the fact that there is no bond between the polyethylene foam and both the inner and outer jackets since they are dissimilar materials. This procedure results in an effective path way between the unbonded dissimilar materials enabling water to migrate up and down the length of the cable in the path way when the outer protective shield of the cable is damaged or a leak occurs relative to the point of cable entry where the hydrophones are attached. Attempted solutions to this problem have proven to be unreliable.
Thus, there is a need for a method of constructing a solid or non-oil-filled hydrophone which does not enable formation of a path way for water to migrate up and down the length of the cable when the outer protective cable shield is damaged or a leak otherwise occurs and which provides a cost effective method and apparatus for a solid hydrophone.
Embodiments of non-oil-filled arrays, and embodiments of equipment and methods of forming non-oil-filled arrays, are described herein. An embodiment of an array provides minimization of cable entry points to one entry point for each channel irregardless of the number of hydrophones used to make up a hydrophone group or array. An embodiment also provides vastly superior protection of the primary cable bundle from damage over present known methods.
In an embodiment, the hydrophone array is provided with uniform buoyancy. Uniform buoyancy is achieved through the use of a hollow micro-sphere loaded Reaction Injection Molded (RIM) polyurethane material. Prior designs have relied on foaming in order incorporate air bubbles into molded areas in the hydrophone and hydrophone cable to achieve buoyancy. The result of prior methods, however, has been a significant variation in the amount of buoyancy achieved. Prior methods have been unable to precisely control the amount of buoyancy in areas of the cable where additional buoyancy is desired. For example, additional buoyancy is desired adjacent the approach to heavier sections of the hydrophone cable where connectors and in the case of digital arrays, the telemetry modules are located.
In one embodiment, a method enables precise adjustment of hydrophone cable buoyancy by providing precise adjustment of the concentration of hollow glass micro-spheres in areas where more or less buoyancy is desired. Use of an Reaction Injection Molding (RIM) process, results in significant reductions in cost and time in the construction of solid towed arrays. The RIM material enables use of a softer, lower durometer matrix in which the micro-spheres reside resulting in a hydrophone cable that is more flexibility than prior known construction methods. For example, the prior known methods provide a minimum bending radius of 6 feet, whereas an embodiment of the method and apparatus provides a minimum bending radius of 18 inches.
An embodiment also provides greater protection of the interior hydrophone cable due to the homogeneous nature of the RIM material (which eliminates the necessity of layering of dissimilar materials and the creation of potential path ways for water to run up and down the hydrophone cable as discussed above) which carries hydrophone signals and array telemetry, resulting in an improvement in the reliability of the system and greater immunity from damage due to abrasions or damage from other sources, for example, shark bite.